Prepurification of gas mixtures before separation thereof by low temperature rectification



A nl 2, 1968 R. BECKER 3,375,674

PREPURIFICATION' OP GAS MIXTURES BEFORE SEPARATION THEREOF BY LOWTEMPERATURE RECTIFICATION Filed Aug. 18, 1966 2 Sheets-Sheet 1 I I I I I.L

RUDOLF BECKER TTORNEY April 2, 1968 R. BECKER 3,375,674

- PHEPURIFICATIQN GAS MIXTURES BEFORE SEPARATION THEREOF TEMP BY 0ERATURE RECTIFICATION Filed Aug. 18, 1966 2 Sheets-Sheet INVENTOR LFBECKER ATTO NEY 3,375,674 Patented Apr. 2 1968 ice L ,4 12 Claims. or.6213) This invention relates to a process for the low temperatureseparation of gaseous mixtures, and in particular to a process whereinCO is removed from raw gas by two simultaneous but separate treatments,one by low temperature congelation, and the other by adsorption, and,more particularly, to the subsequent removal of hydrocarbon impuritiesfrom the resultant Co -free gas.

Processes for the separation of gaseous mixtures are known wherein gasmixture to be separated is warmed to a temperature above the regeneratorexit temperature before being introduced into a turbine wherein it isengine expanded (the term engine expansion meaning expansion with theproduction of external work), in order to provide the refrigerationrequirements of the process. This is done, according to German PatentNo. 1,145,649, for example, by branching off a partial stream from thegas mixture exiting from the cold end of the regenerators, warming thispartial stream in the heat exchange coils provided within theregenerators, and thereafter admixing this partial stream to the coldmain stream.

In another process of this type, described in German Patent No. 833,051,the warming of the gas mixture prior to engine expansion is accomplishedby branching off a partial stream of raw gas incoming from theregenerators at a warm point (above the CO freezing point) andeventually returning same to the main stream exiting from the coolerend. The CO present in the branched-off partial stream is removed in COadsorbers inserted between the point of withdrawal from the regeneratorand the point of admixing with the main stream.

These processes have the disadvantage that the hydrocarbons contained inthe gas mixture enter the expansion engine and the rectification column.The hydrocarbons passing into the sump of the rectification column havea higher boiling point than the oxygen. Therefore, they remain in thesump, are enriched therein, and form an explosive hydrocarbon-oxygenmixture, constituting a constant danger to the entire gas separationplant.

Whereas hydrocarbon adsorbers in gas separation plants are alreadyknown, they are usually inserted in the system in such a manner, forexample in the device described in German Patent No. 1,145,649, thatthey do not prevent the entrance of hydrocarbons, for example,acetylene, into the rectification column. Besides, in most cases,excessive energy is necessary for desorbing these adsorbers, resultingin a disruption of the energy balance of the entire plant. Additionally,special conduits are required for feeding the desorption agent to theadsorber. From the standpoint of hazardous operation, a particulardisadvantage of the conventional plant is that air can condense in thegas phase adsorber and dissolve the hydrocarbons from the adsorptionmedium, and that droplets of high concentration of hydrocarbons may beswept into the turbine or rectification column.

It is, therefore, an object of this invention to provide a lesshazardous process for the adsorption of hydrocarbon impurities from gasmixtures, particularly air.

Another object is to provide such a process which is also economicallyattractive from the standpoint of direct operating costs and plantinvestment costs.

Upon further study of the specification and claims,

other objects and advantages of the present invention will becomeapparent.

For the attainment of the above objects, this invention is brieflydescribed as an improvement in a process of separating gaseous mixturesby low temperature fractionation, which process comprises passin animpure gas mixture containing carbon dioxide and hydrocarbons asimpurities into a cyclically interchangeable heat exchanger and coolingsaid gas mixture therein, a major portion of said gas mixture beingcooled to a sufiiciently low temperature to condense out carbon dioxide,and a minor portion of said gas mixture being removed from said heatexchanger at a point wherein the temperature is above the condensationtemperature of CO passing said removed minor portion through a firstadsorption zone to remove carbon dioxide and produce CO -free minorportion, combining at least part of resultant CO -free minor portionwith CO -free major portion, and fractionating resultant combinedmixture, the improvement comprising the intermediate step of passing thecombined CO -free minor and major portions through a second adsorptionzone to remove hydrocarbon impurities from said combined portions, saidintermediate step being conducted prior to the step of fractionatingresultant combined mixture.

With respect to the attached drawings, FIGURE 1 is a schematicrepresentation of a preferred embodiment of this invention, FIGURE 2 inparticular illustrating the combination of two adsorption zones in oneadsorber.

By means of the improved process of the present invention, the entireamount of the gas mixture can be freed of hydrocarbons in a gas phaseadsorption zone before it enters the rectification column. Furthermore,though branched-off minor portion is first passed through a separate COadsorption zone, the hydrocarbon adsorption zone and the CO adsorptionzone can be arranged so that the same desorption medium stream can beused to desorb both zones. For example, an impure separation product canbe passed successively through both adsorption zones to desorb same,thereby resulting in the avoidance of two separate conduit systems forthe desorption cycle.

Upon recombining the warm CO -free minor portion which amounts to about5 to 20%, preferably 5 to 10% of the original raw gas, with the CO -freemajor portion exiting from the cyclically interchangeable heatexchanger, the temperature of the combined portions is increased to suchan extent that any entrained liquefied gases present in the majorportion is evaporated with certainty before reaching the hydrocarbonadsorber. In this way, the hazard associated with the hydrocarbondissolution action of entrained droplets of liquefied gas is completelyeliminated.

In the case of air separation in particular, the temperature of themajor portion leaving the cyclically interchangeable heat exchangers isabout to K., preferably 105 to 107 K., the temperature of thebranchedoff minor portion about to 150 K., preferably to K., and thecombined CO -free portions prior to hydrocarbon adsorption about 107 to115 K., preferably about 110 K., if the heat exchangers and theadsorbers are operated at a pressure of 8 atmospheres absolute. Otheroperation pressures would require other temperatures.

According to a preferred embodiment of this invention, the twoadsorptive zones can also be superimposed in a single shell, so thatequipment costs are reduced.

When it is desired to subject the gas mixture to an engine expansion,this can be done by means of an expansion turbine or the like which isplaced between the hydrocarbon adsorber and the rectification column. Ahydrocarbon adsorber provided upstream of the expansion turbine can bemore readily desorbed with expanded separation products than an adsorberoperating under a lower pressure, i.e., arranged downstream of theexpansion turbine. This also results in more favorable economics for theprocess.

As a further extension of the invention, a part, about 4 to 10%,preferably 6 to 9% of the starting mixture is branched off from theminor portion after the latter has passed through the CO adsorber, butbefore being combined with the major portion coming from the regeneratoroutlet. This part of the minor portion is expanded to the pressure ofthe low pressure section of a two-stage rectification column, and thenpassed to the low pressure column.

Referring again to FIGURE 1, the preferred embodiment relating to an airseparation plant will now be described in detail. It is to beunderstood, however, that the invention can be employed for theseparation of other gaseous mixtures, particularly where H O or CD andhydrocarbon impurities, are present in the raw gas.

The air to be separated, amounting to about 50,000 Nm. h. is compressedin compressor 1 to about 8 atmospheres absolute and passed to theperiodically reversible regenerators, designated 2, 3 and 14, 15. Aminor portion of this amount of air, about 4,000 Nmfi/h. having atemperature of 135 to 145 K., is branched off from the regenerators at 4and 5, passed through a C adsorber 6 containing, for example silica gel,aluminum oxide, or a zeolite.

The CO -free minor portion is rejoined at junction 7 with the majorportion of the cooled gas mixture to be separated, exiting from theregenerators 2 and 3. The entire quantity of air then flows through thehydrocarbon adsorber 8 which can contain adsorption agents selective forhydrocarbons, for example, silica gel, aluminum oxide gel, or siliciouszeolite. From there, the air is passed through expansion turbine 9wherein the gas is engine expanded to about 5.6 atm. abs., and theresultant expanded gas is passed into the high pressure section of atwo-stage rectification column 10.

At the bottom of the rectification column, an oxygenrich mixture iswithdrawn, further cooled countercurrently against the pure nitrogen gasleaving the low pressure section of the column in the countercurrentsupercooler 11, and, after being expanded to the pressure of about 1.2atm. abs, the pressure maintained in the low pressure section of thecolumn, the resultant liquid-vapormixture is passed into said section.The nitrogen rising from the bottom of the high pressure sectioncondenses in the condenser-vaporizer 13, is withdrawn at that point,passed, for further cooling, through the countercurrent supercooler 12in indirect heat transfer relationship with pure nitrogen gas, andexpanded into the head of the low pressure section. At the top of thelow pressure section, about 40,000 Nm. /h. of pure nitrogen is obtainedand being passed through supercoolers 12 and 11, and then employed forcooling the regenerator 14.

Pure oxygen, in an amount of about 10,000 Nm. /h. can be withdrawn inthe gaseous phase from the sump section of the low pressure section, andis then passed through regenerator 15 in order to cool same.

The dashed lines in the drawing represent the conduit system for thecase wherein a part of the CO -free minor portion exiting from adsorber6 is branched off at junction 16 before being combined with the majorportion. This branched-off part, amounting to about preferably 6-9% ofthe air to be separated, is expanded to the pressure of the low pressurecolumn in the expansion turbine 17, and passed to the low pressurecolumn.

This embodiment of the invention is preferably used in large plants, egfor the production of 30,000 Nm. /h. 0 In smaller plants (capacity e.g.5,000 Nm. /h. 0 only one expansion turbine will be used, providedupstream of the high pressure section of the two stage rectificationcolumn. Of course it would be possible to obtain the necessaryrefrigeration from one turbine also for large plants, but this wouldrequire a markedly increased starting pressure and the energy costs forthe higher pressure of great quantities of air surmount those for theinstallation of a, second turbine.

Referring to FIGURE 2, a preferred embodiment of this invention havingthe two adsorption stages superimposed in a single shell, will now bedescribed in detail.

There is an outer shell 18, preferably provided with a heat insulation,which shell has inlet openings 7 and 21 and outlet openings 16 and 22.If this apparatus were incorporated in the embodiment of'FIGURE 1, theopenings 7, 16, 21 and 22 would be connected to the cold ends ofregenerators 2 and 3, to the expansion machine 17, to the branch points4 and 5. and to the expansion machine 9, respectively. The shell 18comprises an upper portion 8 and a lower portion 6, both filled with anadsorbing agent, such as silica gel, aluminum oxide gel, or molecularsieve. The rooms filled with the adsorbing agent are bordered by sieves19. In order to prevent any direct flowing of gas from the inlet opening7 to the outlet opening 16 there are provided screens 20. With theapparatus shown it is possible to adsorb such substances as water,carbon dioxide and hydrocarbons from two gaseous mixtures which areintroduced into the apparatus via conduits 7 and 21, the first mixturepassing only the upper section and the second one passing both sectionsfrom the bottom to the top, provisions being made for branching off apart of the second mixture via conduit 16.

This apparatus has the definite advantage that its regeneration can beperformed in one step, whereas, if two separate adsorbers were used, tworegeneration steps would be necessary which would desire providing allthe equipment (shell, conduits, etc.) twofold.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Consequently, such changes and modifications are prop erly,equitably, and intended to be within the full range of equivalence ofthe following claims.

What is claimed is:

1. In a process of separating gaseous mixtures by low temperaturefractionating, which process comprises passing an impure gas mixturecontaining carbon dioxide and hydrocarbons as impurities into acyclically interchangeable heat exchanger and cooling said gas mixturetherein, a major portion of said gas mixture being cooled to asufficiently low temperature to condense out carbon dioxide, and a minorportion of said gas mixture being removed from said heat exchanger at apoint wherein the temperature is above the condensation temperature ofCO passing said removed minor portion through a first adsorption zone toremove carbon dioxide and produce CO -free minor portion, combining atleast part of resultant CO -free minor portion with CO -free majorportion, and fractionating resultant combined mixture, the improvementwhich comprises the intermediate step of passing the combined CO -freeminor and major portions through a second adsorption zone to removehydrocarbon impurities from said combined portions, said intermediatestep being conducted prior to the step of fractionating resultantcombined mixture.

2. A process as defined by claim 1 wherein said heat exchanger is aregenerator.

3. .A process as defined by claim 1 wherein impure gas mixture is air.

4. A process as defined by claim 3 wherein said fractionating comprisesrectification in a double column having a high pressure section and alow pressure section.

5. A process as defined by claim 4, further comprising engine expandinghydrocarbon-free combined portions prior to passing same to the highpressure section of the double column.

6. A process as defined by claim 5, further comprising the steps ofengine expanding a part of said CO -free minor portion from said firstadsorption zone, and passing resultant expanded part to the low pressuresection of said double column.

7. A process as defined by claim 6 wherein said heat exchanger is aregenerator.

8. A process as defined by claim 4, further comprising the steps ofengine expanding a part of said Co -free minor portion from said firstadsorption zone, and passing resultant expanded part to the low pressuresection of said double column.

9. A process as defined by claim 1 wherein said fractionating comprisesrectification in a double column having a high pressure section and alow pressure section.

10. A process as defined by claim 9, further comprising engine expandinghydrocarbon-free combined portions prior to passing same to the highpressure section of the double column.

11. Apparatus for separating gaseous mixtures by low temperaturefractionating which mixtures contain carbon dioxide and hydrocarbons asimpurities, which apparatus comprises (a) four cyclicallyinterchangeable regenerators or reversing heat exchangers provided withinlet and Outlet conduits at their warm and cold ends and with branchconduits positioned between the inlet and outlet conduits;

(b) two serially switched adsorption stages, each having an inlet and anoutlet conduit, the inlet conduit of the first of said two stages beingin communication with said branch conduits, the outlet conduit of saidfirst stage being in communication with the inlet conduit of the secondof said two stages, said inlet conduit of said second stage being incommunication with said outlet conduits of said regenerators at theircold ends;

(c) an expansion turbine, having its inlet side in communication withsaid outlet conduit of said second adsorption stage; and

(d) a two-stage rectification column having -a high pressure section anda low pressure section, said high pressure section being incommunication with the outlet side of said expansion turbine and saidlow pressure section being in communication with said inlet conduits ofsaid regenerators at their cold ends.

12. Apparatus for separating gaseous mixtures by low temperaturefractionating which mixtures contain carbon dioxide and hydrocarbons asimpurities, which apparatus comprises (a) four cyclicallyinterchangeable regenerators or re- Versing heat exchangers providedwith inlet and outlet conduits at their warm and cold ends and withbranch conduits positioned between the inlet and outlet conduits.

( b) two serially switched adsorption stages, each having an inlet andan outlet conduit, the inlet conduit of the first of said two stagesbeing in communication with said branch conduits, the outlet conduit ofsaid first stage being in communication with the inlet conduit of thesecond of said two stages, said inlet conduit of said second stage beingin communication with said outlet conduits of said regenerators at theircold ends;

(c) a first expansion turbine, having its inlet side in communicationwith said outlet conduit of said second adsorption stage;

(d) a second expansion turbine having its inlet side in communicationwith said outlet conduit of said first of said two adsorption stages;and

(e) a two-stage rectification column, having a high pressure section anda low pressure section, said high pressure section being incommunication with the outlet side of said first expansion turbine andsaid low pressure section being in communication with said inletconduits of said regenerators at their cold ends and with the outletside of said second expansion turbine.

References Cited UNITED STATES PATENTS NORMAN YUDKOFF, Primary Examiner.V. W. PRETKA, Assistant Examiner.

1. IN A PROCESS OF SEPARATING GASEOUS MIXTURES BY LOW TEMPERATUREFRACTIONATING, WHICH PROCESS COMPRISES PASSING AN IMPURE GAS MIXTURECONTAINING CARBON DIOXIDE AND HYDROCARBONS AS IMPURITIES INTO ACYCLICALLY INTERCHANGEABLE HEAT EXCHANGER AND COOLING SAID GAS MIXTURETHEREIN, A MAJOR PORTION OF SAID GAS MIXTURE BEING COOLED TO ASUFFICIENTLY LOW TEMPERATURE TO CONDENSE OUT CARBON DIOXIDE, AND A MINORPORTION OF SAID GAS MIXTURE BEING REMOVED FROM SAID HEAT EXCHANGER AT APOINT WHEREIN THE TEMPERATURE IS ABOVE THE CONDENSATION TEMPERATURE OFCO2, PASSING SAID REMOVED MINOR PORTION THROUGH A FIRST